1. Field of the Invention
This invention relates to a method for removing prior exposure artifacts in a radiation detection panel and more particularly to a method for operating and reading out information from an X-ray detection panel by continuously cycling the panel and using pre and post exposure information to minimize residual images.
2. Description of Related Art
Direct radiographic imaging using panels comprising a two-dimensional array of minute sensors to capture a radiation generated image is well known in the art. The radiation is imagewise modulated as it passes through an object having varying radiation absorption areas. Information representing an image is captured as a charge distribution stored in a plurality of charge storage capacitors in individual sensors arrayed in a two dimensional matrix.
The panels comprise a two-dimensional array of sensors with associated switching and addressing circuitry built on an insulating substrate, usually a glass plate. Such sensors typically include a pair of generally coplanar conductive microplates separated by a dielectric layer forming a charge storage capacitor. Extending over all the sensors above the microplates is a photoconductive layer which is sensitive to X-ray radiation. A top electrode is placed over the photoconductive layer.
The two microplates in each sensor serve to collect and store charges representing the radiation exposure of the sensor. U.S. Pat. No. 5,319,206 issued to Lee et al. on Jun. 7, 1997, originally assigned to E. I. Dupont de Nemours and Co. Inc. and now reassigned to the assignee of this invention, shows a typical radiation detection panel comprising an array of sensors for the generation and capture of charges following exposure to X-ray radiation.
When using a panel as described in the Lee et al. patent, a charging voltage is applied to the bottom microplates of all sensors and the top electrode. This creates an electric field in the photoconductive layer. Upon exposure to radiation, electron/hole pairs are generated in the photoconductive layer by the absorbed radiation exposure energy.
Under the influence of the applied electric field, the electrons and holes produced separate and migrate toward the top electrode and toward the microplates. In detector structures where a positive charging voltage is applied to the top electrode, electrons move toward the top electrode, and holes migrate toward the top microplates. The hole migration results in a charge accumulation during exposure in the charge storage capacitors formed by the two microplates and the dielectric separating them. Subsequent removal of the charging voltage and the exposing radiation leaves the accumulate charges trapped in the capacitors.
The amount of charge stored in the storage capacitors varies in direct proportion to the radiation exposure. Exposure is normally defined as the product of the radiation intensity "I" times the duration "t" during which radiation impinges on the detector. (E=I.times.t ). The charge Q produced under exposure is given by Q=e.times.(E/.epsilon.), where E is the energy absorbed due to the exposure, .epsilon. is the energy required to generate an electron/hole pair and .epsilon. is the electric charge of an electron.
Readout of the stored charges is accomplished in any one of a plurality of manners. U.S. Pat. No. 5,648,660 by Lee et al. originally assigned to E. I. Dupont de Nemours and currently also re-assigned to the assignee of the present application, discloses a method for the readout of stored charges in a direct radiographic imaging panel.
In reading out information from panels which include a photoconductive layer, one encounters a problem commonly referred to as memory. This memory is the result of a slow, exponential type return of the photoconductor to its original "dark" state following exposure to radiation. Therefore, following each exposure, the photoconductor contains areas having prior image information. When a subsequent exposure of a second image is made, residual image information in the panel produces what is referred to in the trade as phantom or ghost artifacts. Obviously, such artifacts are undesirable as they degrade the image quality of the radiogram and the value of the image for diagnostic purposes.
Attempts to solve this problem include measuring the decay properties of the photoconductor, recording the elapsed time between exposures, and attempting to correct the output of each pixel by subtracting from each output a calculated charge value based on the time elapsed between the prior exposure and the current exposure. Variations of this method are disclosed in U.S. Pat. No. 4,975,935 issued Dec. 4, 1990 to Hillen et al., and U.S. Pat. No. 5,530,238 issued to Meulenbruge et al. Jun. 25, 1996, both assigned to U.S. Philips Corporation. However, this method suffers in that it is based on a calculated value, which is only approximate and which does not accurately reflect the full prior history of the photoconductor area above each sensor, such as the residual effect of multiple exposures at different time intervals.
It is an object of the present invention to provide a method by which the phantom type artifacts discussed above are greatly eliminated and which is not dependent on theoretical computations of the photoconductor decay characteristics.